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Homework Help: Help with parametric equations

  1. Nov 2, 2011 #1
    1. The problem statement, all variables and given/known data
    find parametric equations for the semicircle x^2+y^2=a^2, y>0
    using as parameter the slope t=dy/dx of the tangent to the curve at (x,y)

    2. Relevant equations

    3. The attempt at a solution
    well i know that to parametrize a circle i would use x=acost and y=asint for 0<=t<=2pi
    so i guess if i just want the top half i would use 0<=t<=pi ? but im not sure how to use the derivative of y=sqrt(a^2-x^2) to parametrize the curve
    i went ahead and found dy/dx to be 1/2(a^2-x^2)^(-1/2)*-2x or -x/sqrt(a^2-x^2)
    i dont know what to do next...
  2. jcsd
  3. Nov 3, 2011 #2

    Filip Larsen

    User Avatar
    Gold Member

    Your expression for the slope is correct, but it will probably help you to reduce it a bit more (hint what is a2-x2?).

    If we call the slope for s (our parameter), can you then express x as a function of s and y? If so, perhaps you can use this relation to get rid of x in another equation so it ends up only relating y and s, thus allowing you to find y as a function of s. Repeat with y as a function of s and x to get a parameterization for x.
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